This invention relates to a method and apparatus for changing, with a minimum of disruption, the destination of data messages being transmitted over a single destination data link.
Data messages are used to communicate among the blocks of telecommunication systems. Individual units are frequently removed from service because of trouble or to perform maintenance. If a messaging system, such as the standard LAPD, (Link Access Protocol-D Channel), Messaging System has only a single destination, then the process of switching from one unit to another creates problems because of lost message segments which are in transit to a unit about to be disabled or which form part of an incomplete message sent to a unit being placed in service. A problem of the prior art is that there is no satisfactory arrangement for switching between units without the loss of some of these inter-unit messages if there is a limitation that inter-unit messages have only a single destination.
The above problem is solved and an advance is made over the teaching of the prior art in accordance with this invention wherein when a switch from a controlling unit currently transmitting to a first controlled unit is to be made, such that the controlling unit will subsequently transmit messages to a second controlled unit, the controlling unit stops sending new messages to the first controlled unit and queues additional messages received from clients in its own memory; the controlling unit then sends drain messages to the first controlled unit as a signal that a switch is about the take place; when the controlled unit has acknowledged all of the drain messages sent by the controlling unit, the controlling unit sends a switch command to the first controlled unit; the first controlled unit disconnects from the data link and appropriate data is copied from the first controlled unit to the second controlled unit; the second controlled unit then connects to the data link and the controlling unit sends accumulated and new messages to the second controlled unit which now is the active controlled unit. Advantageously, this arrangement allows for a switch between the first and second controlled unit without the loss of any message segments and with proper acknowledgement of all message segments sent by clients.
In accordance with one aspect of Applicants"" invention, all dynamic memory is copied from the first controlled unit to the second controlled unit before the second controlled unit connects to the data link. Advantageously, this permits all state information and accumulated messages information to be transferred to the controlled unit, so that the controlled unit can process all subsequent message segments.